The present relates generally to an optical fiber scintillation system for use in radiation detecting applications, and more particularly to such an optical fiber scintillation system for on-line, event by event measurement of tritium production.
Tritium, a beta emitting isotope hydrogen, can be produced in several different ways including the neutron irradiation of .sup.6 Li through a .sup.6 Li(n ).sup.3 H reaction in a zero power or a power producing nuclear reactor. The accurate measurement of tritium production is highly desirable. A presently known and commonly used tritium production measuring technique measures the residual helium or tritium in irradiated .sup.6 Li samples as a measure of tritium production, In order to accurately measure such tritium production to an accuracy of 1% by employing such a measuring technique, it was found that at least 10.sup.12 tritium atom-producing n reactions must occur to produce sufficient helium. Because of this requirement, some problems especially those associated with radiation exposure to workers in low power reactor mock-up experiments are present. For example, in a zero power mock-up of a future production reactor expected to replace production reactors of the United States Department of Energy at the Savannah River, S.C. site, the operation of the mock-up at a power of 1 KW in an unshielded facility is needed to provide the number of n reactions necessary for accurate measurements of tritium production by employing the aforementioned measuring technique. The operation of the unshielded reactor mock-up at such high power levels would expose workers to relatively high levels of radiation. In addition to such radiation problems in this reactor mock up, it has been found that in any facility including zero power and power producing nuclear reactors where the irradiation of .sup.6 Li with neutrons is used for tritium production, the provision of a system or technique for measuring or monitoring of tritium production has not been adequately addressed let alone providing such a system or technique capable of on-line, event-by-event measurements of the tritium production.
Accordingly, it is a principal aim or objective of the present invention to provide a optical fiber scintillation system capable of providing on-line, event-by-event measurements of tritium production with such measurements being achievable and accurate to 1% even at reactor power levels that are lower by a factor by at least 10.sup.6 than the 1 KW power levels as would be required using the aforementioned previous technique for measuring tritium production as in the aforementioned zero power mock-up of the production reactor.
Another object of the present invention is to provide such a measuring system having position sensitivity for determining the position in the nuclear reactor or other tritium producer where in the n reactions are occurring and thus providing a positive position sensitive measurement of neutron activity.
Other and further objects of the present invention will become obvious upon an understanding of the illustrative embodiments about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.
In accordance with the present invention, highly accurate on-line tritium production measurements in an event-by-event manner are achieved by coating a scintillation optical fiber of a suitable length with enriched .sup.6 Li, preferably in the form of .sup.6 LiF, or another compound with an index of refraction lower than that of the optical fiber. The .sup.6 Li coated optical fiber is positioned into a neutron producing environment such as found in the core of a nuclear reactor or in another tritium producing facility using neutron irradiation of .sup.6 Li as a tritium producer. Neutrons contacting the .sup.6 Li coating on the fiber effect .sup.6 Li(n )T reactions at the coating with the resulting .alpha. particle or triton (tritium atom) from each reaction depositing some energy in the coating and producing a scintillation light pulse in the underlying fiber. Each light pulse propagates to the end of the fiber where it is amplified and counted by well known conventional scintillation counting techniques. In as much as each n reaction at the .sup.6 Li coating produces a light pulse, and on-line, event-by-event measurement of tritium production is provided. With the .sup.6 Li-coated optical fiber projecting into the reactor, remote measurements of the tritium production, including such measurements as a function of position, can be achieved without encountering radiation exposure problems to workers as heretofore involved.
Lithium glass scintillators employing enriched .sup.6 Li, therein are presently available and have been previously utilized in radiation detectors for various radiation monitoring applications including the detection of alpha, beta, and gamma radiation in extreme environments. These lithium glass scintillators are fabricated by blending selected concentrations of enriched .sup.6 Li with silicate glass. The glass-.sup.6 Li blend is then formed into various shapes of scintillators including disks, rectangles, fibers, or cylinders, or is used in powder form. Such lithium glass scintillators are commercially available from Bicron Corporation, Newbury, Ohio 44065. However, it was found that while such previously available lithium glass scintillators in fiber form could adequately detect neutrons by the .sup.6 Li(n )T reaction, such glass scintillators could not be satisfactorily used for on-line, event-by-event measurements, especially with any position sensitivity, of tritium production in nuclear reactors or other neutron producing facilities. This shortcoming of these previously known lithium glass scintillators is due to the presence of the reactant material, i.e., the enriched .sup.6 Li, in the glass which causes considerable deterioration of the light transmitting characteristics of the glass fiber so as to limit the maximum length of the .sup.6 Li-containing glass fibers to a length of about 2 inches for light transmitting purposes. The use of such short lithium glass scintillation fibers in a reactor or other neutron producing facility for tritium production monitoring purposes would not be practical due to the very limited length of these fibers.
The present invention overcomes the aforementioned problems associated with the limited light propagating characteristics of .sup.6 Li-glass fibers by providing a glass or plastic optical fiber with a .sup.6 Li coating on external surfaces of the fiber. With this external .sup.6 Li coating on the fiber, the fiber can be of any suitable length up to several meters so as to assure a lithium producing zone of neutron activity in a nuclear reactor can be adequately traversed for effecting highly accurate on-line, event-by-event measurements of tritium production.